Woodward, R. C., F. Scherb, F. L. Roesler
As part of its efforts to monitor and characterize periodic variations in the Io plasma torus, the Wisconsin Space Physics group acquired Fabry-Perot spectra of [S II] 6716,6731Å for a five-week period in 1988. Previous analysis (Woodward 1994) showed periodicity, but, surprisingly, not at 10.2 hours--the persistent 3% subcorotational period, often called System IV, reported in other long-term torus data sets. We show here that our 1988 data exhibit a quasiperiodic intensity variation at 10.2 hours, characterized by two distinct intensity enhancements which abruptly changed in relative magnitude during the observational run. We also present perpendicular ion temperature data from the same period, showing a periodic variation anticorrelated with intensity after the aforementioned abrupt change, but no significant periodicity beforehand. This non/anticorrelation appears to eliminate the possibility that a slowly moving magnetic field enhancement is responsible for the subcorotational periodicity. Considering these and other long-term data, we suggest that the various observed subcorotational features are not themselves responsible for the 3% subcorotational period, but rather are independent features modulated or otherwise affected by a permanent, global phenomenon in the torus lagging corotation by 3%.